[Process] Avoid failure when calling Process::stop in edge cases

See https://travis-ci.org/symfony/symfony/jobs/20664994 for an example of failure.
[Process] Avoid failure because of a slow process

See example of failure here https://travis-ci.org/symfony/symfony/jobs/20761834.
[Process] Avoid failures because of slow IOs

See example of failure here https://travis-ci.org/symfony/symfony/jobs/20701462